It is sometimes desirable to increase available power output of existing steam turbines at minimum cost. One method used for this purpose has been to remove at least one stage of blading comprising a stationary blade row and a rotating blade row near the turbine steam inlet in order to allow increased mass flow through the turbine. Depending upon the particular turbine and the desired result, different selected blade rows can be removed from the turbine. For example, removal of the second stage blading will allow an increase in pressure drop across the first stage blading and a concomitant increase in mass flow. The increased mass flow increases the available power output from the turbine, albeit at a slightly reduced efficiency.
In the past, removal of blade rows has entailed machining away of the airfoil section of each blade in a blade row. The blades may be from either stationary or rotating blade rows or combinations of both stationary and rotating blade rows. Rotating blade rows may be machined off at the blade platform and stationary blades may be machined off at their roots. Alternately, each blade may be removed in its entirety. In either case, removal of at least the airfoil section of the blades has resulted in discontinuities or uneven surfaces where the blades were removed. In general, where blades are machined off, the inner surface of the turbine cylinder is not smooth and may include labyrinth seals and attendant supports for stationary blades. When steam flows over these uneven surfaces, turbulence and flow separation occur which further reduces turbine efficiency. It is therefore desirable to reduce such turbulence and flow separation in order to minimize efficiency decreases when turbines are operating with less than the designed number of blade rows.